Page 329 - Color Atlas of Biochemistry

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320 Tissues and organs

Ethanol metabolism The rate of ethanol degradation in the liver
is limited by alcohol dehydrogenase activity.
+
The amount of NAD available is the limiting
A. Blood ethanol level
factor. As the maximum degradation rate is
Ethanol (EtOH, “alcohol”) naturally occurs in already reached at low concentrations of
fruit in small quantities. Alcoholic drinks ethanol, the ethanol level therefore declines
containmuchhigher concentrations. Their at a constant rate (zero-order kinetics). The
–1
alcohol content is usually given as percent calorific value of ethanol is 29.4 kJ g .Alco-
by volume. To estimate alcohol uptake and holic drinks—particularly in alcoholics—can
the blood alcohol level, it is useful to convert therefore represent a substantial proportion
the amount to grams of ethanol (density of dietaryenergyintake.
–1
0.79 kg L ). For example, a bottle of beer
(0.5 L at 4% v/v alcohol) contains
20 mL = 16 g of ethanol, while a bottle of C. Liver damage due to alcohol
wine (0.7 L at 12% v/v alcohol) contains Alcohol is a socially accepted drug of abuse in
84 mL = 66 g ethanol. Western countries. Due to the high potential
Ethanol is membrane-permeable and is for addiction to develop, however, it is ac-
quickly resorbed. The maximum blood level tually a “hard” drug and has a much larger
is already reached within 60–90 min after number of victims than the opiate drugs, for
drinking. The resorption rate depends on var- example. In the brain, ethanol is deposited in
ious conditions, however. An empty stomach, membranes due to its amphipathic proper-
a warm drink (e. g., mulled wine), and the ties, and it influences receptors for neuro-
presence of sugar and carbonic acid (e. g., in transmitters (see p. 352). The effect of GABA
champagne) promote ethanol resorption, is enhanced, while that of glutamate declines.
whereas a heavy meal reduces it. Ethanol is High ethanol consumption over many
rapidly distributed throughout the body. A years leads to liver damage. For a healthy
largeamountis taken up by themuscles and man, the limit is about 60 g per day, and for
brain, but comparatively little by adipose tis- a woman about 50 g. However, these values
sue and bones. Roughly 70% of the body is are strongly dependent on body weight,
accessible to alcohol. Complete resorption of health status, and other factors.
the ethanol contained in one bottle of beer Ethanol-related high levels of NADH+H +
(16 g) by a person weighing 70 kg (distribu- and acetyl-CoA in the liver lead to increased
tion in 70 kg 70/100 = 49 kg) leads to a synthesis of neutral fats and cholesterol.
blood alcohol level of 0.33 per thousand However, since the export of these in the
(7.2 mM). The lethal concentration of alcohol form of VLDLs (see p. 278) is reduced due to
is approximately 3.5 per thousand (76 mM). alcohol, storage of lipids occurs (fatty liver).
This increase in the fat content of the liver
(from less than 5% to more than 50% of the
B. Ethanol metabolism
dry weight) is initially reversible. However, in
The major site of ethanol degradation is the chronic alcoholism the hepatocytes are in-
liver, although the stomach is also able to me- creasingly replaced by connective tissue.
tabolize ethanol. Most ethanol is initially oxi- When liver cirrhosis occurs, the damage to
dized by alcohol dehydrogenase to form etha- the liver finally reaches an irreversible stage,
nal (acetaldehyde). A further oxidization, cata- characterized by progressive loss of liver
lyzed by aldehyde dehydrogenase, leads to ace- functions.
tate. Acetate is then converted with the help
of acetate-CoA ligase to form acetyl CoA,using
ATP and providing a link to the intermediary
metabolism. In addition to cytoplasmic alco-
hol dehydrogenase, catalase and inducible
microsomal alcohol oxidase (“MEOS”; see
p. 318)alsocontributetoalesserextenttoetha-
nol degradation. Many of the enzymes men-
tioned above are induced by ethanol.

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